Technology | Sep 21, 2007

Toward pure white light: Next-generation LEDs show bright promise

Scientists in India are reporting an advance toward discovering a Holy Grail of the illumination industry — a white LED, a light-emitting diode that produces pure white light suitable for interior lighting of homes, offices and other buildings. Their study is in the Sept. 9 issue of ACS' The Journal of Physical Chemistry C, a weekly publication ("White Light from Mn2+-Doped CdS Nanocrystals: A New Approach").

D. D. Sarma and Angshuman Nag point out that practical versions of these so-called white LEDs would be brighter, longer-lasting and more energy efficient than conventional light sources such as incandescent and fluorescent lamps and could replace them in the future. However, scientists have faced several difficulties in developing pure white LEDs with all the requirements and desirable properties. Existing versions produce tinted, unstable shades of white light that mar their performance.  

 
In their publication the scientists showed that the broad PL spectra of the new CdS NCs can be tuned by controlling the dopant concentration, and white lights of different shades can be produced. Mn2+-doped CdS NCs (0.10 and 0.19%) produce white lights with chromaticity coordinates (0.30, 0.40) and (0.35, 0.40), which is within the white region of the 1931 CIE diagram. Because of a huge stokes shift between the absorption and emission spectra the well-known self-absorption problem is minimized, resulting in no change in the emission spectrum of a given sample either in solution or in the solid state. Their conclusion was that these NCs can be excited over a wide range of excitation wavelengths without compromising the chromaticity. 

With these results the researchers report the first success in developing a new LED based on a new phosphor from semiconductor nanocrystals of cadmium sulfide mixed with manganese. It produces a stable shade of white light that remains constant over time and appears superior in overall performance in comparison to previous generations of white LEDs. The scientists now are working to boost its efficiency so that the white LED can be used in everyday applications  

Reedited Source: American Chemical Society

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